The invention relates to a glassmaking furnace regenerator simultaneously providing a device for trapping impurities present in the gaseous effluents in order to reduce the emission of dust.
Most so-called flame glassmaking furnaces, that is to say those having gas or oil burners as the source of energy, are equipped with regenerators. The regenerators comprise a succession of chambers filled with ceramic members forming a stack and recovering and restoring the heat in cycles. The hot gases or flue gases coming from the operating furnace enter the stack, generally at the top of the regenerator, and deliver their heat energy into the stack. During this time, cool air is fed into the bottom of another stack heated during the preceding cycle to recover the heat energy; that air is hot when it leaves the top of the stack, from which it is conveyed to the burners of the furnace in order to ensure combustion of the fuel under the best possible conditions.
Furnaces for melting glass produce large quantities of gaseous effluents. Dust is formed in these effluents as they are cooled between the inlet and the outlet of the regenerators. The term xe2x80x9cdustxe2x80x9d refers to particulate emissions, that is to say any organic or inorganic substance in solid form (with no size limit) or in liquid form (droplets).
The dust essentially arises from the condensation of chemical species coming from the evaporation and recombination of substances present in the bath of glass during manufacture. It is also caused by the presence of impurities in the fuels. To a lesser degree, it also comes from flying raw materials in solid form.
The person skilled in the art knows that some of the dust, in greater or lesser amounts depending on the operating conditions of the furnace, can be deposited on entering the regenerators. Such deposits tend to block the passages for the flue gases and air. Until now, designers have always attempted to design stacks that limit this blocking phenomenon.
FR-A-2756820 proposes a method of selectively heating conventional stacks in order to unblock parts blocked by deposits of dust.
At present, the applicable standards in many countries are aimed at regulating and increasingly severely reducing particulate emissions.
Accordingly, glass manufacturers are obliged to study new ways to reduce particulate emissions from furnaces for melting glass.
Several solutions are used at present.
That most widely used is the electrostatic precipitator. This type of device collects the great majority of particulate emissions, but has serious drawbacks. The investment and operating costs of such equipment are very high. Also, the equipment is rapidly damaged by the acid gases and treating these gases before they enter the filter is therefore recommended, which represents an additional constraint.
Bag filters and, more generally, membrane filters are also used. They also collect the great majority of particulate emissions but have the same drawbacks as the device described above. Moreover, this type of filter operates at a low temperature, which necessitates cooling the flue gases before treating them in the filter. Finally, adding an auxiliary filter system leads to problems with controlling the operation of the furnace, because of the resulting head losses.
There is therefore a requirement for a device for reducing the dust emitted in the flue gases from glassmaking furnaces that is efficient and does not have the drawbacks of existing systems.
The invention aims to satisfy this requirement by proposing a regenerator that simultaneously acts as a device for trapping impurities present in the gaseous effluents from glassmaking furnaces and thereby limits the formation of dust and thus recourse to an auxiliary filter system.
The requirement is satisfied by a regenerator stack having an arrangement that favors, optimizes and controls condensation of the species generating dust on the surface of the refractory parts constituting the stack, which obviously retains its heat exchanger function.
To be more precise, the invention provides a glassmaking furnace regenerator including a stack of several rows of refractory members defining a plurality of channels, characterized in that the stack comprises, in the hot gas flow direction, a first zone at the hot gas inlet, for rapidly cooling the hot gases, a second zone, or central zone, for condensing and trapping chemical species liable to generate dust, and a third zone, at the cooled gas outlet, for evacuating condensates, the rows of stacked refractory members constituting said central zone including at least two adjacent rows whose channels have a projected surface area at least 20% less than those of the channels of the first and third zones.
The expression xe2x80x9cprojected surface area of the channelsxe2x80x9d means the largest surface area delimited by the refractory walls as seen in plan view. For example, reducing the size of the channels or offsetting the channels over two successive rows reduces the projected surface area of the channels. The projected surface area reduction must be at least 20% for a significant effect to be obtained.
By xe2x80x9crowxe2x80x9d is meant a storey or layer of stack members.
The stack of this regenerator is advantageously formed, at least in part, by electrofused cruciform stackable members.
For research purposes, we have developed a device for evaluating the quantity of dust contained in the flue gases on an industrial site. It comprises a water-cooled stainless steel sampling pipe. It extracts in an isokinetic manner a representative sample of the flue gases flowing in the stacks under controlled sampling conditions. The solid particles are collected on a filter and the gas flow passes through a series of washing flasks containing appropriate absorption solutions. Analyses of the filtrate and the washing solutions quantify the dust concentration already formed and the concentrations of species in the form of vapor liable to generate dust. The device can be used throughout the range of temperatures and speeds encountered in the stacks and therefore to monitor changes between the top and the bottom of a regenerator chamber.
Studying the dust collected on the filter has confirmed that the dust from glassmaking furnaces comprises very small particles (size less than one micrometer), of which by far the greatest part consists of sodium sulfate (especially if the glass being manufactured is an alkali-lime glass). The sodium sulfate is the result of the reaction between Na2O in the vapor state (resulting from the evaporation of species from the bed of raw materials and above the bath of glass itself) and SO2 from the fuel and the raw materials. It condenses at temperatures below approximately 1 100xc2x0 C. and then solidifies when the flue gases are cooled to below approximately 900xc2x0 C. These steps occur during cooling of the flue gases and therefore in the regenerators, when regenerators are used.
The SO2 and the sodium oxide are in gaseous form at the stack inlet. Because of the cyclic operation of the stack, the temperature of the refractory materials is lower than that of the flue gases. A temperature gradient is therefore established between the flue gases at the center of the channel and the flue gases in contact with the refractory walls. As soon as the temperature of the stack members falls below the temperature at which sodium sulfate condenses, condensation on the surface of the refractory members begins. Then, when the temperature of the flue gases becomes substantially equal to the temperature at which sodium sulfate condenses, the latter condenses spontaneously in the form of a mist at the center of the channel. Some of the droplets constituting that mist are deposited on the refractory walls. When the temperature of the flue gases falls below the temperature at which sodium sulfate solidifies, the droplets change from the liquid state to the solid state.
Our measurements on various industrial furnaces have enabled us to devise and validate the above dust formation mechanism.
Consequently, we are essentially interested in sodium sulfate dust; the phenomena demonstrated also occur for other species (sulfates, chlorides, borates of alkali and alkaline-earth metals), although at significantly different temperatures.
It therefore appeared to us that, to reduce the amount of dust emitted when using regenerator stacks, the stacks must be capable of maximizing the condensation on the refractory materials constituting the stack of chemical species liable to generate dust.
The object of the invention is to provide different solutions enabling this objective to be achieved. Unlike conventional regenerators, in which the whole of the stack is entirely for transferring heat, stacks in accordance with the invention all have three zones, each of which has a specific role. The first zone encountered by the flue gases is a zone aiming to condition the flue gases so that the efficiency of the central zone is maximized. Thus it is a question in particular of maintaining the species generating dust in vapor form and obtaining a flue gas temperature greater than but close to the temperature at which these species condense. This fast thermal conditioning must leave more room for the central zone in which the trapping phenomena actually occur. The various possible configurations of the central zone are intended to intensify the condensation and then deposition of droplets on the surface of the stack members.
The third and final zone is designed to facilitate the natural or forced evacuation of condensates and other deposits flowing by gravity from the preceding zones.
The stacks of the invention can be made with all conventionally used types of sintered or fused refractory members. The examples given in this application refer to various cruciform electrofused members for filling regenerator chambers manufactured and sold by the Soci xc3xa9txc3xa9 Europxc3xa9enne des Produits Rxc3xa9fractaires (SEPR). These products are perfectly suitable for this application because they feature a very low reactivity vis xc3xa1 vis alkaline vapor contained in the flue gases and are very resistant to high temperatures and to thermal cycling imposed by the operation of the regenerators. Also, the specific surface area of heat exchange of these members is high, which favors heat exchange and achieves very high regeneration efficiency. Also, the electrofused refractory material constituting the cruciform members from SEPR is particularly able to withstand cleaning the stacks by heat treatment.
The following description and the various diagrams explain the invention in more detail.